Keep in mind wastewater and fresh tanks are NOT designed to be heavy enough to pressurize much or draw a vacuum on them. When mounted for use, they are supported from the bottom by the pan, and usually supported on the sides by the rest of the enclosure or frame members.
I would fill them up to the max once supported as installed and the outlet is plumbed, and then just look for any leaks after they sit overnight...
Rich, KE4GNK/AE, Overkill Engineering Dept.
'The Silver HamShack' ('07 International 22FB CCD 75th Anniversary)
Multiple Yaesu Ham Radios inside and many antennae sprouting from roof, ProPride hitch, Prodigy P2 controller.
2012 shortbed CrewMax 4x4 Toyota Tacoma TV with more antennae on it.